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What is the difference between byte streams (InputStream/OutputStream) and character streams (Reader/Writer) in the classic java.io package? How do you read all bytes or all lines of a small-to-medium text file in a single, concise call using modern NIO.2 utility methods? How does wrapping a FileInputStream in a BufferedInputStream (or FileReader in a BufferedReader) improve I/O performance, and what's the mechanism behind this improvement? How would you implement object serialization using ObjectOutputStream/ObjectInputStream, and what does the Serializable marker interface and serialVersionUID actually do? How would you use try-with-resources combined with multiple chained I/O wrapper classes (like FileInputStream wrapped in BufferedInputStream wrapped in GZIPInputStream) to correctly read a compressed file while ensuring all layers are properly closed? How would you use the WatchService API (java.nio.file) to monitor a directory for file system changes (creation, modification, deletion) in real time? What is the purpose of the PrintStream and PrintWriter classes (like System.out), and how do they differ from lower-level OutputStream/Writer in their exception-handling behavior? How would you recursively copy an entire directory tree (including all subdirectories and files) using the Files.walkFileTree() method and a FileVisitor implementation? How does java.nio's Channel and Buffer abstraction differ fundamentally from the classic java.io Stream model, and what specific capability does this enable for non-blocking I/O? How would you implement memory-mapped file I/O using MappedByteBuffer, and what performance advantage does this provide for very large files compared to traditional stream-based reading?
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What are the modern java.nio.file utility classes (Path, Files) introduced in Java 7 (NIO.2), and how do they improve on the older java.io.File class?

Intermediate

Answer

Path represents a file system location (replacing the older File class's more limited abstraction), and the Files utility class provides a comprehensive set of static methods (readAllLines(), copy(), move(), exists(), walk() for recursive directory traversal, and many more) covering essentially all common file operations far more completely and with better error reporting than the old File class ever offered (File's methods often just silently returned false on failure with no indication of WHY, while Files' methods throw a specific, informative IOException) -- NIO.2 is now the generally recommended, more modern and capable API for virtually all file system interaction in contemporary Java code.
Path path = Path.of("data", "config.txt");

// Comprehensive, well-designed utility methods with informative exceptions
List<String> lines = Files.readAllLines(path, StandardCharsets.UTF_8);
Files.copy(path, Path.of("backup", "config.txt"), StandardCopyOption.REPLACE_EXISTING);

// Recursive directory walk, returning a lazily-evaluated Stream
try (Stream<Path> walk = Files.walk(Path.of("."))) {
    walk.filter(Files::isRegularFile).forEach(System.out::println);
}
Real-world example A file-organizing utility uses Files.walk() combined with Stream filtering to recursively find and process all matching files in a directory tree, a task that would have required significantly more manual recursive code using the older File class's more limited listFiles()-based API.

Common follow-ups: Why does Files.delete() throw a specific exception on failure while File.delete() just silently returns false?;How does Path's relationship to the java.nio.file.FileSystem abstraction enable working with non-default file systems (like a ZIP file treated as a file system)?

Error Handling;Java Networking & HTTP Client
All 15 I/O & NIO questions